1. Field of the Invention
This invention relates to a color cathode-ray tube (CRT) and, more particularly to a color CRT including a tension focus mask.
2. Description of the Background Art
A color cathode-ray tube (CRT) typically includes an electron gun, an aperture mask, and a screen. The aperture mask is interposed between the electron gun and the screen. The screen is located on an inner surface of a faceplate of the CRT tube. The screen has an array of three different color-emitting phosphors (e.g., green, blue, and red) formed thereon. The aperture mask functions to direct electron beams generated in the electron gun toward appropriate color-emitting phosphors on the screen of the CRT tube.
The aperture mask may be a focus mask. Focus masks typically comprise two sets of conductive electrodes (or wires) that are arranged approximately orthogonal to each other, to form an array of openings. Different voltages are applied to the two sets of conductive electrodes so as to create quadrupole focusing lenses in each opening of the mask. The quadrupole focusing lenses are used to direct the electron beams toward the color-emitting phosphors on the screen of the CRT tube.
The electron beams that are directed onto the color-emitting phosphors through the aperture mask may have either a positive tolerance or a negative tolerance. An incident electron beam whose cross-section is larger than the area of its corresponding effective color-emitting phosphor element is characterized as being a negative tolerance system. A negative tolerance tube typically has opaque matrix lines separating the phosphor elements, wherein the phosphor elements are located in matrix openings located between the matrix lines. An incident electron beam whose cross-section is smaller than the area of its corresponding effective color-emitting phosphor element is characterized as a being a positive tolerance system. In a positive tolerance system the beam cross-section essentially defines the width of the effective phosphor element. The term effective phosphor element as used in this disclosure refers to the stripe width of the phosphor stripe that a viewer can observe. In conventional negative tolerance tubes, the width of the phosphor element equals the width of the matrix opening.
Additionally, the electron beams that are directed onto the color-emitting phosphors through the aperture mask may also have clipping tolerances as well as leaving tolerances. The term clipping tolerance refers to the smallest distance that the electron beam can move relative to the screen before the electrons in the incident electron beam start to excite an adjacent (incorrect) color-emitting phosphor element. For negative tolerance CRTs, leaving tolerance refers to the smallest distance that the edge of an electron beam can move relative to the screen before part of a phosphor element is no longer excited by electrons in the incident electron beam. In positive tolerance CRTs, leaving tolerance refers to the smallest distance an edge of the beam must move to begin missing part of the effective phosphor element.
Conventional negative tolerance aperture mask CRT""s typically have clipping and leaving tolerances that are built into each tube and may be only slightly adjusted. For example, for a CRT having a 0.68 mm screen pitch, the magnitude of adjustment for either of the clipping or leaving tolerances is typically less than about 0.025 mm (1 mil). Also, generally adjustments to either of the clipping and leaving tolerances, inversely affect the other. For example, increasing the leaving tolerance of a CRT will decrease the clipping tolerance thereof, and vice versa. This inverse relationship is true unless the phosphor elements are made smaller. In such case, the clipping and leaving tolerances can both be increased; however, this done at the expense of light output.
Thus, a need exists for cathode-ray tubes (CRT) with large clipping and leaving tolerances without sacrificing light output.
The present invention relates to a method of operating a cathode-ray tube (CRT) having a focus mask in a positive tolerance mode, wherein the CRT has an evacuated envelope with an electron gun therein for generating at least one electron beam. In the preferred embodiment, the envelope further includes a faceplate panel having a luminescent screen with only color-emitting phosphor stripes on an interior surface thereof. The color-emitting phosphor stripes each have a width slightly smaller than one-third of the screen pitch, such that large clipping and leaving tolerances may be achieved therewith. The focus mask has a plurality of spaced-apart first conductive electrodes and is located adjacent to an effective picture area of the screen. The spacing between the first conductive electrodes defines a plurality of slots substantially parallel to the color-emitting phosphor stripes on the screen. Each of the first conductive electrodes has a substantially continuous insulating material layer formed on a screen facing side thereof. A plurality of second conductive electrodes (wires) are oriented substantially perpendicular to the plurality of first conductive electrodes and are bonded thereto by the insulating material layer. The method comprises directing electron beams from electron guns in the envelop toward the color-emitting phosphors on the screen and selectively applying different voltages to the two sets of conductive electrodes such that clipping and leaving tolerances of the CRT are selectively increased.